An image sensor according to the related art has a pixel structure including a photodiode and a transistor. In the pixel having the above structure, an incident light excites electrons of the photodiode so that a current flows. The quantity of the current varies according to the intensity of the light. Accordingly, after a predetermined time has elapsed, a shutter switch is turned on, and signals are transferred to a sense node so that image signals can be obtained.
However, the signal level is determined according to the capacity of a condenser of the sense node. Since the capacity of the condenser of the sense node is fixed in the related art, the operating range of an imaging device is narrowed. In addition, since the imaging device cannot perform an automatic exposure function, the imaging device performs the exposure function by adjusting the exposure time of a sensor or adding the gain of an image signal through an image signal processing block.
In order to solve the above problems, there has been suggested a technology in which a varactor employing a MOS capacitor is used so that the capacity of a condenser of the varactor can be adjusted to expand the operating range of the imaging device.
However, since a voltage level range of the storage region including the varactor in an image sensor employing the varactor differs from the range in which the capacitance of the varactor actually varies, the dynamic range cannot be actually expanded.